Android动态库的加载system.loadlibary（三）

System.loadlibary，System.load是怎么加载so库的？他们是在Runtime的启动过程中通过loadnativelibary来加载libjavacore.so的动态库，这个动态库是这两个函数的实现体。

libcore/ojluni/src/main/java/java/lang/System.java

    public static void loadLibrary(String libname) {
        Runtime.getRuntime().loadLibrary0(VMStack.getCallingClassLoader(), libname);
    }

直接调用了Runtime的函数loadLibary0，然后经过doload,nativeload进入native层，

libcore/ojluni/src/main/native/runtime.c

JNIEXPORT jstring JNICALL
Runtime_nativeLoad(JNIEnv* env, jclass ignored, jstring javaFilename,
                   jobject javaLoader, jstring javaLibrarySearchPath)
{
    return JVM_NativeLoad(env, javaFilename, javaLoader, javaLibrarySearchPath);

}

接着通过jvm的LoadNativeLibary执行实际工作，

art/runtime/openjdkjvm/openjdkjvm.cc

JNIEXPORT jstring JVM_NativeLoad(JNIEnv* env,
                                 jstring javaFilename,
                                 jobject javaLoader,
                                 jstring javaLibrarySearchPath) {
  ScopedUtfChars filename(env, javaFilename);

  std::string error_msg;
  {
    art::JavaVMExt* vm = art::Runtime::Current()->GetJavaVM();
    bool success = vm->LoadNativeLibrary(env,
                                         filename.c_str(),
                                         javaLoader,
                                         javaLibrarySearchPath,
                                         &error_msg);
  }

  // Don't let a pending exception from JNI_OnLoad cause a CheckJNI issue with NewStringUTF.
  env->ExceptionClear();
  return env->NewStringUTF(error_msg.c_str());
}

art/runtime/java_vm_ext.cc

bool JavaVMExt::LoadNativeLibrary(JNIEnv* env,
                                  const std::string& path,
                                  jobject class_loader,
                                  jstring library_path,
                                  std::string* error_msg) {
  error_msg->clear();

  // See if we've already loaded this library.  If we have, and the class loader
  // matches, return successfully without doing anything.
  // TODO: for better results we should canonicalize the pathname (or even compare
  // inodes). This implementation is fine if everybody is using System.loadLibrary.
  SharedLibrary* library;
  Thread* self = Thread::Current();
  {
    // TODO: move the locking (and more of this logic) into Libraries.
    MutexLock mu(self, *Locks::jni_libraries_lock_);
//判断是否已经加载过这个ｌｉｂａｒｙ
    library = libraries_->Get(path);
  }
  .......
  // Open the shared library.  Because we're using a full path, the system
  // doesn't have to search through LD_LIBRARY_PATH.  (It may do so to
  // resolve this library's dependencies though.)

  // Failures here are expected when java.library.path has several entries
  // and we have to hunt for the lib.

  // Below we dlopen but there is no paired dlclose, this would be necessary if we supported
  // class unloading. Libraries will only be unloaded when the reference count (incremented by
  // dlopen) becomes zero from dlclose.

  Locks::mutator_lock_->AssertNotHeld(self);
  const char* path_str = path.empty() ? nullptr : path.c_str();
  void* handle = android::OpenNativeLibrary(env,
                                            runtime_->GetTargetSdkVersion(),
                                            path_str,
                                            class_loader,
                                            library_path);

//是否需要ｎａｔｉｖｅｂｒｉｄｇｅ的帮助，如果ｈａｎｄｌｅ为空指针，说明前面OpenNativeLibrary失败了。
  bool needs_native_bridge = false;
  if (handle == nullptr) {
    if (android::NativeBridgeIsSupported(path_str)) {
      handle = android::NativeBridgeLoadLibrary(path_str, RTLD_NOW);
      needs_native_bridge = true;
    }
  }

?　// Create a new entry.
  // TODO: move the locking (and more of this logic) into Libraries.
  bool created_library = false;
  {
    // Create SharedLibrary ahead of taking the libraries lock to maintain lock ordering.
    std::unique_ptr new_library(
        new SharedLibrary(env, self, path, handle, class_loader, class_loader_allocator));
    MutexLock mu(self, *Locks::jni_libraries_lock_);
    library = libraries_->Get(path);
    if (library == nullptr) {  // We won race to get libraries_lock.
      library = new_library.release();
      libraries_->Put(path, library);
      created_library = true;
    }
  }

  bool was_successful = false;
  void* sym;
  if (needs_native_bridge) {
    library->SetNeedsNativeBridge();
  }
  sym = library->FindSymbol("JNI_OnLoad", nullptr);
  if (sym == nullptr) {
    VLOG(jni) << "[No JNI_OnLoad found in \"" << path << "\"]";
    was_successful = true;
  } else {
    // Call JNI_OnLoad.  We have to override the current class
    // loader, which will always be "null" since the stuff at the
    // top of the stack is around Runtime.loadLibrary().  (See
    // the comments in the JNI FindClass function.)
    ScopedLocalRef old_class_loader(env, env->NewLocalRef(self->GetClassLoaderOverride()));
    self->SetClassLoaderOverride(class_loader);
  }

  library->SetResult(was_successful);
  return was_successful;
}

LoadNativeLibrary的目的就是通过各种手段去加载动态链接库，然后执行其中的JNI_OnLoad接口（这个函数是jni库的首选入口，可以利用它完成一些初始化工作）。

上面函数中有个NativeBridge的概念：这个技术用来处理当前系统平台的指令集和目标对象(Library)的指令集不一致时的兼容性问题。比如说，Android的模拟器，通常直接编译成x86版本运行在开发机器上，那么对于一个只提供了arm版本的动态链接库文件的apk，如何保证它可以在x86模拟器上运行，就是nativeBridge要解决的问题。

正常途径，加载Library利用的是OpenNativeLibrary这个函数。

system/core/libnativeloader/native_loader.cpp

void* OpenNativeLibrary(JNIEnv* env,
                        int32_t target_sdk_version,
                        const char* path,
                        jobject class_loader,
                        jstring library_path) {
#if defined(__ANDROID__)
  UNUSED(target_sdk_version);
  if (class_loader == nullptr) {
    return dlopen(path, RTLD_NOW);
  }

  std::lock_guard guard(g_namespaces_mutex);
  android_namespace_t* ns = g_namespaces->FindNamespaceByClassLoader(env, class_loader);

  if (ns == nullptr) {
    // This is the case where the classloader was not created by ApplicationLoaders
    // In this case we create an isolated not-shared namespace for it.
    ns = g_namespaces->Create(env, class_loader, false, library_path, nullptr);
    if (ns == nullptr) {
      return nullptr;
    }
  }

  android_dlextinfo extinfo;
  extinfo.flags = ANDROID_DLEXT_USE_NAMESPACE;
  extinfo.library_namespace = ns;

  return android_dlopen_ext(path, RTLD_NOW, &extinfo);
#else
  UNUSED(env, target_sdk_version, class_loader, library_path);
  return dlopen(path, RTLD_NOW);
#endif
}

先利用FindNamespaceByClassLoader查找当前的ClassLoader是否有相关的Namespace，如果没有直接跳转到android_dlopen_ext；如果有调用其Create方法创建一个Namespace。

android_dlopen_ext跟dlopen类似，第一个参数是要打开的动态库的名称，第二个参数RTLD_NOW，表示动态库中所有未定义的符号在dlopen返回前都会被解析。

接下来的实现，是调用find_libary来查找动态库，找到后，调用dlsym来查找加载的动态库中是否包含JNI_OnLoader入口函数。

 

接下来看下Java层声明的native函数如何与本地层的具体实现建立其正确的映射关系。

在动态库的入口函数JNI_OnLoader中，通常会调用AndroidRuntime::registerNativeMethods方法：

int ret1 = AndroidRuntime::registerNativeMethods(env,
                   "android/media/ImageReader", gImageReaderMethods, NELEM(gImageReaderMethods));

registerNativeMethods就是把JNI中的函数注册到虚拟机中，这个函数是对JNIRegisterNativeMethods的中转。

libnativehelper/JNIHelp.cpp

extern "C" int jniRegisterNativeMethods(C_JNIEnv* env, const char* className,
    const JNINativeMethod* gMethods, int numMethods)
{
    JNIEnv* e = reinterpret_cast(env);
    scoped_local_ref c(env, findClass(env, className));

    if ((*env)->RegisterNatives(e, c.get(), gMethods, numMethods) < 0) {
        char* tmp;
        const char* msg;
        if (asprintf(&tmp, "RegisterNatives failed for '%s'; aborting...", className) == -1) {
            // Allocation failed, print default warning.
            msg = "RegisterNatives failed; aborting...";
        } else {
            msg = tmp;
        }
        e->FatalError(msg);
    }

    return 0;
}

变量env是JNIEnv对象，是由JNI_OnLoader的JavaVM获取的，并且是线程私有的。

art/runtime/jni_internal.cc

  static jint RegisterNatives(JNIEnv* env, jclass java_class, const JNINativeMethod* methods,
                              jint method_count) {
    return RegisterNativeMethods(env, java_class, methods, method_count, true);
  }

 static jint RegisterNativeMethods(JNIEnv* env, jclass java_class, const JNINativeMethod* methods,
                                    jint method_count, bool return_errors) {
    if (UNLIKELY(method_count < 0)) {
      JavaVmExtFromEnv(env)->JniAbortF("RegisterNatives", "negative method count: %d",
                                       method_count);
      return JNI_ERR;  // Not reached except in unit tests.
    }
    CHECK_NON_NULL_ARGUMENT_FN_NAME("RegisterNatives", java_class, JNI_ERR);
    ScopedObjectAccess soa(env);
    mirror::Class* c = soa.Decode(java_class);
    if (UNLIKELY(method_count == 0)) {
      LOG(WARNING) << "JNI RegisterNativeMethods: attempt to register 0 native methods for "
          << PrettyDescriptor(c);
      return JNI_OK;
    }
    CHECK_NON_NULL_ARGUMENT_FN_NAME("RegisterNatives", methods, JNI_ERR);
    for (jint i = 0; i < method_count; ++i) {
      const char* name = methods[i].name;
      const char* sig = methods[i].signature;
      const void* fnPtr = methods[i].fnPtr;
      if (UNLIKELY(name == nullptr)) {
        ReportInvalidJNINativeMethod(soa, c, "method name", i, return_errors);
        return JNI_ERR;
      } else if (UNLIKELY(sig == nullptr)) {
        ReportInvalidJNINativeMethod(soa, c, "method signature", i, return_errors);
        return JNI_ERR;
      } else if (UNLIKELY(fnPtr == nullptr)) {
        ReportInvalidJNINativeMethod(soa, c, "native function", i, return_errors);
        return JNI_ERR;
      }
      bool is_fast = false;
      // Notes about fast JNI calls:
      //
      // On a normal JNI call, the calling thread usually transitions
      // from the kRunnable state to the kNative state. But if the
      // called native function needs to access any Java object, it
      // will have to transition back to the kRunnable state.
      //
      // There is a cost to this double transition. For a JNI call
      // that should be quick, this cost may dominate the call cost.
      //
      // On a fast JNI call, the calling thread avoids this double
      // transition by not transitioning from kRunnable to kNative and
      // stays in the kRunnable state.
      //
      // There are risks to using a fast JNI call because it can delay
      // a response to a thread suspension request which is typically
      // used for a GC root scanning, etc. If a fast JNI call takes a
      // long time, it could cause longer thread suspension latency
      // and GC pauses.
      //
      // Thus, fast JNI should be used with care. It should be used
      // for a JNI call that takes a short amount of time (eg. no
      // long-running loop) and does not block (eg. no locks, I/O,
      // etc.)
      //
      // A '!' prefix in the signature in the JNINativeMethod
      // indicates that it's a fast JNI call and the runtime omits the
      // thread state transition from kRunnable to kNative at the
      // entry.
      if (*sig == '!') {
        is_fast = true;
        ++sig;
      }

      // Note: the right order is to try to find the method locally
      // first, either as a direct or a virtual method. Then move to
      // the parent.
      ArtMethod* m = nullptr;
      bool warn_on_going_to_parent = down_cast(env)->vm->IsCheckJniEnabled();
      for (mirror::Class* current_class = c;
           current_class != nullptr;
           current_class = current_class->GetSuperClass()) {
        // Search first only comparing methods which are native.
        m = FindMethod(current_class, name, sig);
        if (m != nullptr) {
          break;
        }

        // Search again comparing to all methods, to find non-native methods that match.
        m = FindMethod(current_class, name, sig);
        if (m != nullptr) {
          break;
        }

        if (warn_on_going_to_parent) {
          LOG(WARNING) << "CheckJNI: method to register \"" << name << "\" not in the given class. "
                       << "This is slow, consider changing your RegisterNatives calls.";
          warn_on_going_to_parent = false;
        }
      }

      if (m == nullptr) {
        LOG(return_errors ? ERROR : INTERNAL_FATAL) << "Failed to register native method "
            << PrettyDescriptor(c) << "." << name << sig << " in "
            << c->GetDexCache()->GetLocation()->ToModifiedUtf8();
        // Safe to pass in LOG(FATAL) since the log object aborts in destructor and only goes
        // out of scope after the DumpClass is done executing.
        c->DumpClass(LOG(return_errors ? ERROR : FATAL), mirror::Class::kDumpClassFullDetail);
        ThrowNoSuchMethodError(soa, c, name, sig, "static or non-static");
        return JNI_ERR;
      } else if (!m->IsNative()) {
        LOG(return_errors ? ERROR : FATAL) << "Failed to register non-native method "
            << PrettyDescriptor(c) << "." << name << sig
            << " as native";
        ThrowNoSuchMethodError(soa, c, name, sig, "native");
        return JNI_ERR;
      }

      VLOG(jni) << "[Registering JNI native method " << PrettyMethod(m) << "]";

      m->RegisterNative(fnPtr, is_fast);
    }
    return JNI_OK;
  }

注册过程的目的是建立Java函数和native函数之间的对应关系，所以先通过FindMethod找到Java_class中的Java层函数，这个注册通常是一个函数集合methods，集合中的每一个元素都是JNINativeMethod对象，包含了java函数名、signature、native函数原型。FindMethod查找到的结果是一个ArtMethod，最后有RegisterNative完成注册工作，它要做的就是将native method在内存中地址通过SetEntryPointFromJni设置为对应ArtMethod的JNI入口。在JNI环境下，当java层的函数被调用后，首先找到它的ArtMethod对象，然后通过GetEntryPoint得到JNI入口代码的地址。

这就完成了java层跟native层函数的JNI映射。